Recent developments in information and communication industry enable compact, lightweight, thin, and portable electronic devices, and thus high energy densification of batteries used as power supplies of such electronic devices are increasingly demanded. A lithium secondary battery may be a most suitable one to satisfy such demands, and thus studies on the lithium secondary battery are being actively carried out.
Carbonaceous materials are generally used as an anode material of a lithium secondary battery, and the carbonaceous materials include crystalline carbon and amorphous carbon. Representative examples of crystalline carbon may include graphite carbon, such as natural graphite or artificial graphite; and examples of amorphous carbon may include non-graphitizable carbon (hard carbon) obtained by carbonization of a polymer resin and graphitizable carbon (soft carbon) obtained by heat treatment of pitch.
In general, soft carbon is made by applying 1,000 levels of heat to coke which is a by-product produced during crude oil refining, and exhibits high output and short charging time unlike a conventional graphite anode active material or hard carbon-based anode active material.
On the other hand, hard carbon may be produced by carbonization of a material such as resin, thermosetting polymer, or wood. When such hard carbon is used as an anode material of a lithium secondary battery, it has a high reversible capacity of 400 mAh/g or more due to micropores but has low initial efficiency of approximately 70%. Thus, it is disadvantageous in that when hard carbon is used for an electrode of a lithium secondary battery, irreversible consumption of lithium is significant.
Such irreversibility is observed because solid electrolyte interphase (SEI) as a surface film is created by dissociation of electrolyte on the surface of an electrode during charging, or because lithium stored in carbon particles during charging is prevented from being discharged during discharging. Of these two cases, the former case is more problematic and the creation of a surface film is known as a major cause of irreversibility.
Moreover, it is known that most of high-capacity graphite materials have a highly developed layer structure, and thus have a high degree of graphitization and a flake shape. In the case of such flake-shaped graphite, regions where Li ions are intruded between the layers thereof, that is edge surfaces, are small. Thus, when the flake-shaped graphite is used as an anode active material of a lithium secondary battery, a high rate discharge characteristic, which is a characteristic in the case of discharge with high current, is deteriorated.
Furthermore, spherical natural graphite is disadvantageous in that it has a limited ionic conductivity, and empty spaces are created between active materials to increase the resistance of an electrode when only the spherical natural graphite is used as an anode active material, thereby bringing about a decrease in rate performance.
Therefore, it is necessary to develop an anode active material capable of replacing typical anode active materials, and reducing interfacial resistance and improving rate performance when applied to a lithium secondary battery.